
DEPARTMENT OF COMMERCE 



.:^ '7,'^ ^\>>'s.;'i5V"t;?i?3i- 



Scientific Papers 

OF THE 

Bureau of Standards 

S. W. STRATTON, Director 



No. 397 

a study of the relation between the 

brinell hardness and the grain size 

of annealed carbon steels 



HENRY S. RAWDON, Physicist 

Bureau of Standards 

EMILIO JIMENO-GIL, Professor of Physical Chemistry 

University of Oviedo, Oviedo, Spain 



SEPTEMBER 20, 1920 




PRICE, 10 CENTS 

Bold only by the Superintendent of Documents, Govermnent Printing Office 

Washington, D.C. 



WASHINGTON 

GOVERNMENT PRINTING OFFICE 

1920 



DEPARTMENT OF COMMERCE 



Scientific Papers 

OF THE 

Bureau of Standards 

S. W. STRATTON. Director 



No. 397 

A STUDY OF THE RELATION BETWEEN THE 

BRINELL HARDNESS AND THE GRAIN SIZE 

OF ANNEALED CARBON STEELS 



HENRY S. RAWDON, Physicist 

Bureau of Standards 

EMILIO JIMENO-GIL, Professor of Physical Chemistry 
University of Oviedo, Ouiedo, Spain 



SEPTEMBER 2U, ly2U 




PRICE. 10 CENTS 

Sold only by the Superintendent of Documents, Government Printing Oaic. 
Washington. D. C. 

WASHINGTON 

GOVERNMENT PRINTING OFFICE 

1920 



<^. 



'^3 



<^ 



n/.rj/r'A 



^0 3 



A STUDY OF THE RELATION BETWEEN THE BRINELL 
HARDNESS AND THE GRAIN SIZE OF ANNEALED 
CARBON STEELS 

By Henry S. Rawdon and Emilio Jimeno-Gil 



CONTENTS Page 

I. Introduction 557 

1 1 . Materials and method 55^ 

III. Microscopic examination 560 

IV. Hardness determinations 562 

V. Factors affecting hardness 578 

\'I. Grain growth upon annealing after cold working 585 

\'ll. Summary 59^ 

I. INTRODUCTION 

It is a matter of common agreement amon.i.^ metallurgists and 
users of metals in general that the "grain size" of metals and 
alloys is a factor of fundamental importance in detennining the 
characteristic properties of the material. There is, however, no 
such common agreement as to which of the mechanical properties 
is most profoundly affected by differences in grain size, or to what 
extent the grain size may be used as a measure of such properties. 
Much of the distrust with which a coarsely granular metal, ]3ar- 
ticularly steel, is received is largely a matter of the combined ex- 
periences common to all metallurgists rather than to any particular 
investigation along this line. The subject of grain size has re- 
ceived a very considerable amount of attention. The extensive 
stucUes of Jeffries stand foremost '■ in the list. 

Howe - and Gulliver ' have also made noteworthy contributions 
to the subject. In these studies attention was directed principally 
to the conditions necessary for the occurrence of grain growth and 
to the means of measuring such grov>'th. The relation between 
grain size and mechanical properties, such as are measiu^ed in the 
ordinary' methods of testing, was not made the primary object of 
the studies. Most of the work on grain size in metals has l^een 

' Two of the most important references to tliis author's work arc: Z. Jeffries. Grain-GrowthPhenomena 
in Metals, Bull. Am. Inst. Min. Engrs.. .5(i. p. 571, 1916, and EITect of Temperature. Defomiatiuii, and 
Grain Size on the Mechanical Properties of Metals, Am. Inst. Min. Engrs., 1919. Previous articles by the 
same author are summarized and discussed in full in the second article. 

- H. M. Howe, Grain Growth. Bull. Am. Inst. Min. Engrs., 56, p. 582; 1916. 

3 G. H. Gulliver, Grain Size, Jour. Inst. Metals, 19, No. i. p. m-;: 191S. 

SS7 



558 Scioitific Papers of the Bureau of Standards \Voi.i6 

based also upon materials of relatively simple structure; for in- 
stance, simple metals and one-constituent alloys. Among the few 
attempts made to correlate the results of grain-size determinations 
with the mechanical properties of the metal may be mentioned 
the rather comprehensive study by Bassett and Davis * upon a 
brass. It was shown that for brasses of the same composition 
which had previously received a similar mechanical treatment a 
rather definite relation exists between the Brinell hardness and 
the average size of crystals comprising the material. In dis- 
cussing this work Mathewson ^ has shown that a formula may be 
derived which expresses this relationship rather closely. But lit- 
tle work, however, has been done to establish such a relationship 
for steels, if such a one does exist. The studies made by McAdam 
upon the grain size of soft steel ^ have been in the direction of 
defining the conditions by which grain growth may occur in such 
material rather than in developing the relationship of the proper- 
ties of the material to the grain size of the same. The work of 
Pomp ' which appears to be the most comprehensive study in 
this field, was restricted to only one type of material, a steel of 
0.08 per cent carbon content being used throughout. While such 
a steel permits of an accurate grain size determination more 
readily than do others higher in carbon, a knowledge of mechanical 
properties of this material is of much less industrial importance 
than of the steels of higher carbon content. The work herein dis- 
cussed was undertaken to determine to what extent the mechanical 
hardness of steels of different carbon contents is dependent upon 
grain size and to show also what other factors are contributory. 
It is hoped that the work will be extended later to cover the rela- 
tion of grain size to the mechanical properties of steels other than 
hardness. 

II. MATERIALS AND METHOD 

The general plan of study included the determination of the 
hardness of specimens which were of the same composition but 
differed widely in their grain size. The material used comprised 
five steels of the composition shown in Table i. Since it was 
desired to express the grain size numerically, if possible, the dif- 
ferent steels were chosen with this in view. Hence material of 
approximately eutectoid composition was avoided. 

* W. H. Bassett and C. H. Davis. Comparison of Grain-Size Measurements and Brinell Hardness of 
Cartridge Brass, Bull. Am. Inst. Min. and Met. Engrs., p. 692: 1919. 

^ C. H. Mathewson. Discussion of Bassett and Davis's paper. 

«D.J. JIcAdam, jr.. Proc. A. S. T. M.. 17. Pt. II. p. 5S: 1917. Also, 18. Pt. II, p. 6S; 191S. 

^ A. Pomp, Einfluss der War-mbehandlung auf die Kerbzahigkeit, Komgrusse and Harte von kohlen 
stoffannem Flusseisen. Ferrum, 13. p. 49; 1915-16. 



Hardness a)id Grain Size oj Steels 
TABLE 1. — Results of Chemical Analyses of Steels Usea " 



559 



Specimen 


Carbon 


Manganese 


Phosphorus 


Sulphur 


Silicon 




Per cent 

0,07 
.19 
.46 
.70 


Per cent 

0. 27 
.41 
.36 
.22 
.23 


Per cent 

0.006 
,004 
.019 
.023 
.019 


Per cent 

054 
.050 
.047 
.011 
.013 


Per cent 


B 








D 




E 











« Much ol the material was furnished through the courtesy of the Carnegie Steel Co. 

Ill order to develop grains varying widely in size, two dilTerent 
methods were adopted. In the first the specimens (two-inch lengths 
of bars, one by one-half inch cross section) were heated for six 
honrs at various temperatures, as shown in Table 2. Two speci- 
mens of each composition were heated at each of the temperatures 
indicated ; one was cooled slowly in the f luiiace at the close of the 
six-hour period, the other allowed to cool more rapidly in the air. 
For each particular temperature the specimens were all heated 
together (likewise cooled together) in order to eliminate possible 
effects from variations in heating conditions. For heating, an 
electric-resistance furnace of the muffle type was used for the fom- 
lower temperatures, and the higher temperatures were obtained in 
a small gas-fired muffle furnace. The temperature measurements 
were made by means of a chromel-alumel thermocouple and a small 
portable potentiometer. In order to avoid decarburization during 
the continued heating at the high temperatures (974, 1024, and 
1112° C, Table 2), the specimens were packed in a mixture of 
amorphous silica and 5 per cent of powdered charcoal. This was 
found, by trial, to be very effective in retarding decarburization; 
only a relatively thin layer was affected. When the specimens 
were prepared for the microscopic examination and the determina- 
tion of hardness, they were groimd deeply enough to remove 
entirely the decarburized layer. 

The second method for producing variations in grain size con- 
sisted in annealing bars after they had been given a preliminary 
cold working by stressing in tension. This is discussed in detail 
in Section VI. 



560 Scientific Papers of the Buveaii of Standards [Voi.16 

TABLE 2. — Annealing Temperatures and Methods of Cooling Specimens 



Designation of specimens 


Temperature for 6 hours 


Method ; 
of cooling 


Remarks 






Degrees centigrade 










674 (672-676) 


In air...' 




nal 








grain size preserved 








In fur- 


Do 


















In air .. 


Just above Ai transformation; oi 
nal grain size replaced by a fi 


ei 








ner 








one 
















































nace 










In air. 
In fur- 
















nace 






2-A-A, 2-B-4, 2-C-4, 2-D-4, 


974 (952-995) 


In air.... 






2-E-4 






Above Aa transformation 




2-E-44 




















l-E-5 










l-A-55, l-B-55, l-C-55, l-D-55. 


1024 (1010-1038) 








l-E-55 










A-5, B-5, C-5, D-5, E-5 


1112 (1098-1126) 


In air.... 






A-55, B-55, C-55, D-55, E-55... 


1112 (1098-1126) 


In fur- 






A-66, B-66, C-66, D-66. E-66.. - 


754° for 35 minutes; 674 


Quenched 


Grain refinement by **double 


an- 




{672-6761 for 6 hours 


in water, 
reheated 

and 
cooled 
in fur- 


nealing" 





III. MICROSCOPIC EXAMINATION 

The specimens were examined microscopically and a grain size 
determination Avas attempted. The method used was the plani- 
metric one, modified and described by Jeffries " and adopted by 
the American Society for Testing Materials in its tentative speci- 
fications for the preparation of micrographs.'' 

In Table 3 are summarized the results of the grain count. Only 
for the steels of the low-carbon content (particularly those cooled 
in air) and heated at the highest temperatures could a satisfactory 
grain-size determination be made Avith certainty. 



» Z. Jeffries, Am. Inst. Min. Engrs.. .)4. p. ;t,j; 1916. Also. Jlet. and Chem. Eng.. 18, p. 1S5: 191; 
' Tentative Spec. E r-19 T. Proc. A. S. T. :M.. 1!>, Pt. I, p. ;-o; 1919. 



Hardness a)id Grain Size of Steels 
TABLE 3.— Results of Grain-Size Determination 



561 



Average 



A-I... 

A-2... 
A-3... 
A-4-.. 
2-A-4. 
l-A-5. 
A-5... 

B-1... 
B-2... 
B-3... 
B-4 ... 

2-B-4. 
l-B-5. 
B-5... 

C-1... 
C-2... 
C-3... 
C-4... 
2-C-4. 
l-C-5. 
C-5... 

D-1.. 
D-2.. 
D-3.. 
D-4.. 
2-D 4 
l-D-5 
D-5.. 

E-1.. 
E-2.. 
E-3.. 
E-4.. 
2-E-4 
l-E-5 
E-5.. 



1024 
1112 



1024 
1112 



1024 
1112 



1690 
2330 

2010 
1330 



1930 
2610 



(») 

(") 

(") 

2410 
45 250 
49 880 
77 820 

(") 



47 620 



A-U... 
A-22... 
A-33... 
A^4... 
2-A-44. 
1-A-5S. 
A-55... 

B-U... 
B-22... 
B-33... 
B-44... 
2-B-44. 
l-B-55- 
B-55.-. 

C-11... 
C-22-.. 
C-33... 
C-44... 
2-C-44. 
l-C-55. 
C-55... 

D-11.. 
D-22.. 
D-33.. 
D-44.. 
2-D-44 
l-D-55 
D-55.. 

E-11.. 
E-22.. 
E-33.. 
E-44-. 
2-E-44. 
l-E-55 
E-55.. 



tempera- 
ture of 
heating 


Number 
of grains 
per mm2 


°C 




674 


1800 


762 


(") 


849 


9.in 


953 


(") 


974 


92 


1024 


66 


1112 


32 


674 


2857 


762 


(") 


849 


360 


953 


290 1 


974 


220 


1024 


168 


1112 


144 


674 


(") 


762 


1") 


849 


(■'I 


953 


(") 


974 


19 


1024 


1") 


1112 


7,5 


674 


(") 


762 


r'l 


849 


1") 


953 


("1 


974 


("1 


1024 


23 


1112 


U 


674 


1") 


762 


(") 


849 


{"1 


953 


116 


974 


49 


1024 


34 


1112 


20 



10 920 
15 200 
31 640 



2810 
3410 
4490 
5950 
6940 



8660 
20 410 
29 411 
49 140 



^ No satisfactory count could be made. 

On account of tliis uncertainty, therefore, there are given in 
Figs. I to II, incKisive, typical micrographs of the specimens to 
show the structural condition which resulted from the treatment 
which the material received. They also illustrate the difficulties 
encountered in making an accurate grain-size determination, as 
well as the structural features other than grain size which affect 
the properties nf the material. In Figs. 12 and 13 are given 
typical microgra])hs of the steels heated at the highest tempera- 



562 Scientific Papers of the Bureau of Standards iVoi.i6 

tare, liighlv enough magnified to demonstrate the effect of the 
rate of coohng upon the structure of the steel; that is, the inner 
structure of the grains. 

The specimens of series i (A-i, . . . . E-i and A-ii . . . . 
E-i i), which were heated to a temperature sHghtly below the Acj 
transformation retain the original grain size of the material. The 
specimens heated just above this transformation show a decrease 
in grain size (in those specimens in which the grains are clearly 
defined). This is best seen in the materials which were cooled 
in the air. In all of the five types of steels the increase in grain 
size is very slight for the lower temperatm-es. Not until the Acg 
transformation has occurred and the steel is in the gamma phase, 
is the increase in the grain size very marked. In some of the 
steels this occurs upon heating when the temperature of the Acg 
transformation is reached. In two, however (0.46 and 0.70 per 
cent carbon) , no appreciable increase in grain size occurs until the 
temperature is considerably above that of the Acg transformation 
(approximately 200°). The marked increase for these specimens 
occurs also within a relatively very narrow range of temperature, 
950 to 975° C, approximately. 

In Fig. 14 there is given a portion of the iron-carbon constitu- 
tional diagram, in which has been indicated, for piuposes of refer- 
ence, the range of temperature in which the grain size increases 
most rapidly in each of the steels used. 

IV. HARDNESS DETERMINATIONS 

Two methods were used for determining the mechanical hard- 
ness of the material. The specimen used for the determination 
of the microstructure was large enough (i by 1^2 inches face) to 
permit all the determinations of hardness to be made upon the 
one piece. The two forms of the Brinell hardness-testing set 
used will be referred to as the "standard " and the " micro-Brinell " 
set, respectively. For the former the usual hydraulic type was 
used, and a load of 500 kg was applied to the specimen by means 
of a ball 10 mm in diameter for 30 seconds. For the second set 
of determinations a small "dead-weight" type of Brinell appa- 
ratus was used. The instrument (Fig. 1 5) , which was developed 
by the Ordnance Department of the United iStates Army, was 
loaned for the purpose. A load of 15 kg for 30 seconds vipon a 
ball one-sixteenth inch in diameter was used. The details of 
construction and manipulation have been described elsewhere.^" 

'» S. h. Gondale and R M. Banks. Development of Brinell Harne!,s Tests on Thin Sheet Brass, Proc. 
A.. S. T. .M . 19. PI, n. p. 757; itjig. 



HavJiicss and Giaiii Size of Steels 



563 







'II 



«•>■ 



<>- 



^-V-- U 



Ml X'"^ 

/OZ4-' 

■ ^ V 



Img. i.'-MiaoAxuciioc of o.u; l^o tcut cathon 6Lcl (.^Av/ A) aflc 
aiincaliiuj and coohiuj i)i ait; X75 {reduced from X-^oo) 

liach specimen was hcatcLi six hours at temperature (°C) shown. Hl<-hed wit] 
2 per cent alcohohc nitric acid 



564 



Scientific Papers of the Bureau of Standards 



[Vol. 16 







> ■ ' 


'■ ", --' ' ' .' 1 ■ %' ■C'- 

OTA" 






9S3 - 




'■ '■ '' * 




A 


ji4\~X&-^:^'^^M:2,:^i 






76 Z' 


97^° 




r ~ 


"" 


^ . '' '■'- V ". : 


/ 
f 


^ 




> • ■ 


\ 


^a24-'' 




^ 


^'' 




/IZO" 





I'lG. 2. — Microstritciurc of o.oy per cent carbon steel (steel A) after 
annealing and eooliuq 1)1 the furnace; y^y^ {reduced from \ioo) 



Each specimen was heated si 
2 per cent alcoholic nitric acid 



i at temperature (" C) shown. Etched with 



Rawdon 
J„ne„o-G 



Hardness and Grain Size of Steels 



565 














XC^V- 


*'/'-''' 




S7^° 




^ 




•"— A. . 1 












Fig. 3. — Micros! met ure of o.ig per cent carbon steel {^leel B) after 
annealing and cooling in air; X/S (f educed from Xioo) 

Each specimen was heated six hours at temperature ("C) shown. Etched with 
; per cent alcoholic nitric acid 



566 



Scioitific Papers of tlic Bureau of Standards 




KiG. 4. — MicwsliHituic of O.I Q per ecu I carbon slccl htci I B) after annealing 
and cooling in furnace; X/5 {reduced from )\Ioo) 

Each specimen was heated six hours at temperature (° C) shown. Ktched with 2 per 
cent alcoholic nitric acid 



Hardness a)id Grain Size of Steels 



567 




Fig. ^.—Micoitniiiiin- of i> 46 pc> cent onhoii steel {sled C] after auneaUnq 
and eooliinj in aii: ■ y^ iie<liie,,l fieiii ■ /ok) 

liacll specinic;u was licju-.l bix liuurs at teniperaUire (° C) ^lloWIl. Utched with 2 per 
cent alcoholic nitric acid 



568 



Scientific Papers of the Btircau of Standards 




Fig. 6. — Micioslructurc of 0.46 per cent carbon steel {steel C) after aniifuliiKj 
ai}d cooling in furnace; XyS {reduced from Xjoo) 

Each specimen was heated six hours at temperature C*C) shown. Etched with j per 
cent alcoholic nitric acid 



Raudon 1 



Hardness and Grain Size of Steels 



569 




Fig. 7. — Microstructure of O.yo per cent carbon steel (ik'el D) after auuMtliihi 
anil cooling in aii; X75 (reduced fiom X^ou) 



Each specimen \v 
cent alcoholic nitric 



temperature (° C) shown. Etched with 2 per 



570 Scientific Papers of the Bureau of Standards \Vv. 




Fig. 8. — Mtcroitnictuic of «.yo per cent carbon steel {steel D) after ainiealhig 
and cooling in furnace; X/i {reduced from Xioo) 

Each specimen was heated six hours at temperature (° C) shown. Etched with 2 per 
cent alcoholic nitric acid 



Rawdon "1 
Jimeno-Gili 



Hardness and Grain Size of Steels 



571 




and coollny in air; -'75 Voluccd from Kioo) 

^ u = hPitM six hours al lemperaturc (°C) shown. Etched with .- per 

Each specimcu was heated six nours ai L^ii^n 

cent alcoholic nitric acid 
198S°— 'JO 3 



572 



Scientific Papers of the Bureau of Standards [Voi.id 




Fig. io. — Microstnicture of I.I3 per cent carbon steel (steel E) after auncaUng 
and cooling in furnace; y^y ^ {reduced from y^ioo) 



Each specimen was heated s 
alcoholic nitric acid. 



b at temperature ( ' C ) sho 



Etched with 2 per cent 



'j^nenZciil Hcudiuss and Grain Size of Steels 



573 



l.-'-V' 



r- 



/i 



'i^*. 




."^ 



:c^::^ 






""""""i 



£ 1 

J. II. — Mictoiliuctiiix of the five types of steel used, after maximum 
uain nfincmcnt. Magnification of micrograph'! at left, X75 (''f''""''^ 
row >Uoo); at right X375 {rcducea from X500) 



Fig. 11.— Mio OS 
gta 
fro 



jrom :>^iuu), ui nyiu ^j/j i,/tui..,-v.j.,^... y-^j — , 

A specimen of each of the steels was quenched in water after being heated 
7-4° C. it was then heated for 6 hours at 574° C. Etched with 2 per cei 



nitric acid 



'hca in water alter oeing neaiea 3 s niinutca 
574° C. Etched with 2 per cent alcoholic 



574 



Scientific Papers of the Bureau of Standards 



[ Vol. :6 



The Brinell hardness numlier was calculated by means of the 
ordinary formula; 



Pressure 



area of spherical indentation irtD 

in which / = K (D — -^D' — d-),D and d being the diameter of the 
ball and of its indentation, respectively. Although the loads 





£ 



Fig. i:?. — Effect of rate of cooling upon the structure of steels B, C, ami E: 
X3T5 (reduced from X500) 



These were cooled in air after a period of heatii 
cent alcohoUcnllric acid. (Compare Fig. ii) 



of six hours at ii ij^C. Etched with 2 per 



used for the two Brinell sets were not in exactly the same ratio 
as the sizes of the two balls, the hardness numbers obtained by 
the two are in very fair agreement. 

The results of the determinations of hardness are summarized 
in Tables 4 and 5. The comparative hardness of the various 



Rawdon "1 
Jvneno-Git\ 



Hardness and Grain Size of Steels 



575 



steels in different conditions of strncture are best sliown, however, 
in the graphical results of Figs. i6 to 19, inclusive. In plotting 
the diagrams the temperatiu-es at which the various steels were 





Miil/k/mi.^ 



Fir,. i^.^F.ffai of rale of cooliii:/ iifoii III, .sIniLliin of^UriiB, C an.ll-. 
X.)75 (reduced from Kjoo) 



These were cooled in the furnace after a period of lieatingof six hours at iiij°C. liuhed 
with 2 percent alcoholic nitric acid. (Compare Fis: 12.) Note the character of tlie pearlite 
and the widtli of the fcrrile envelopes) 

treated in order to develop the different grain size, ratlier than 
the numerical measures of the grain-size determinations, were 
used as abscissas. 



576 Scientific Papers of the Bureau of Sta.7idards 





uV 






V 






® 


® 


®® 


e^\^ 


• 




.2 1 


a 


® 


® . 


• 


; 




4 t 
? ^ 

» V 

®« 


® 

® 
® 


® 

® 
® 


® . 

®® 
® . ^ 


• 


/ 

^ 


• 
« 




• 

• 
• 



Joi 



Ftg. 14. — Poriiou of the iron-carhon constitutional diiupat. 



The temperature ran^ein which pronounced grain growth of the various steels (Table 1) 
■cciirred in a period ot si-\ hf)urs has been indicateil 




Fig. 15. — Apparatus for obtaining micro-Brinell hanini.\^s niimbe 



Hanliiess and Giaiii Size of Steels 



577 



TABLE 4. — Micro-Brinell Hardness Numbers of Alr-Cooled and Furnace-Cooled 

Specimens " 



Specimen 


Hardness number 


Specimen 
furnace cooled 


Hardness number 


air cooled 


MaximumMinimum 


Average '' 


Maximum 


Minimum 


Average '' 


A 1 


83 76 


79 
78 
85 
82 
86 
72 
82 

88 

103 
105 
102 
103 
87 
90 

153 
162 
173 
177 
185 
164 
177 

169 
194 
199 
201 
213 
193 
200 

151 
212 
225 
259 
270 
236 
260 


A-11 

A-22 


76 
75 
83 
73 
83 
70 
70 

91 
87 
87 
85 
93 
79 
80 

150 
139 
145 
146 
164 
135 
148 

167 
164 
175 
165 
170 
163 
170 

140 
165 
182 
194 
191 
191 
195 

75 
95 
150 
164 
156 


69 
71 
77 
70 
76 
61 
66 

88 
84 
83 
83 

81 
72 
75 

148 
132 
140 
142 
157 
130 
138 

160 
156 
166 
161 

148 
152 
163 

135 
162 
179 
186 
185 
ISO 
191 

74 
91 






81 
8S 
83 
88 
74 
84 

91 
107 
107 
106 
105 
89 
93 

157 
164 
174 
180 
191 
167 
185 

171 
198 
202 
204 
219 
202 
208 

155 
218 
228 

267 
279 
240 
264 


82 
81 
El 

81 

86 
99 

103 
95 

101 
85 
88 

144 
151 
170 
172 
180 
161 
165 

166 
186 
194 
197 
208 
191 
195 

148 

208 
220 
256 
262 
231 
257 






A-33 

A-44 

2-A-44 

l-A-55 

A-55. 

B-11 

B 22 












l-A-5 


65 






B-2 






B-33 

B-44 

2-B-44 

l-B-55 

B-55 

C-U 




B-4 


84 


l-B-5 




B-5 


77 






C-3.... 


C-33 


143 




2-C-44.... 


161 




132 




C-55 


146 




D-ll 


164 








D-33 












2-D-44 

l-D-55 

D-55 

E-U 

E-22 

E-33 


159 




155 


D 5 

E-1 


165 
137 




164 








E-44 


190 










l-E-55 


184 




E-55 


193 




A-66 


74 










B-66 


93 








C-66 










D-66 


159 ' 162 






E-66 


153 155 


1 









' 15 kg load applied to one-sbcteenth-inch ball for ^o seconds. 
' Average of 6 determinations. 



578 



Scientific Papers of the Bureau of Sfandards [Vot.,6 

TABLE 5.— Standard Brinell Hardness Numbers « 



Specimen, 
air cooled 


Hard- 
number' 


Specimen, 
furnace cooled 


Hard- 

number i 


Specimen, 
air cooled 


H|yl- 1 Specimen, 
numberli,'"™^"'^'""^'! 


Hard- 

number^ 




75 
78 
81 
81 
81 
72 
80 

89 
95 
95 
96 
98 
83 
93 

124 
149 
160 
161 
170 
159 
176 




73 
72 
7! 
69 
65 
62 
64 

85 
82 
76 
76 
74 
73 
76 

140 
122 
129 
129 
143 
120 


D 1 








A-2 


A 22 


D 2 .. 


182 
185 
188 
198 
184 
194 

130 
198 
212 
237 
258 
227 
232 


D 22 








D-3 

j D 4 






A-4 










2-A-44 

l-A-55 

A 55 




2-D-44 

l-D-55 

D-55 










A 5 


D 5 




B 1 


B-11 


E-1 












B-3 


B 33 








B 4 


B 44 . .. 


E 4 


E^44 




2 B-4 


2-B-44 

l-B-55 

B 55 




2-E-44 

l-E-55 

E 55 




l-B-5 . . 


1-E 5 




B 5 


E 5 




C 1 


C 11 
















C-3 


C-33 






C-4 


C 44 






2 C-4 


2-C-44 

l-C-55 






1 C 5 





















'I 5CO kg load applied to lo mm ball for .^o seconds. 
f> Average of j readings. 

V. FACTORS AFFECTING HARDNESS 

From a comparison of llie results of the deterininations of hard- 
ness as summarized in Figs. i6 to 19 with the micrographs showing 
the structure of the materials resulting from the different treat- 
ments, it is evident that no simple direct relation between grain 
size and Brinell hardness exists for carbon steels as does for some 
alloys; for instance, alpha l)rass. Grain size is a matter of minor 
importance v\-ith respect to hardness, as compared wdth some of 
the other factors involved. 



Rawdon I Hanhicss a}!(I Grai)! Size of Siccis 

jimifu^CilA 



579 




j,.,r,. ,-_-^Mu:,o-Hn„cUh,u,l,u.> of ll„- Jr. .>l.:h .,/!,, .„un> .,:o„lh 

■ ' , , 1 ,,h„„rcat thctcmijcraturein.liiateduml tui.lediu theluruace 

Kach specimen was heated mx hourt at llic icmpLiaim^ 



58o 



Scientific Papers of the Bureau of Standards 



— w 



S 


Per- 
centage 
diBer- 
encea 


-H „ ^ c-> 1> O O 


~n 


" 


< 


as 

0" 


;;3 ^ 2 ?^ ::^ =^ s 


stand- 
ard 

Brinell 
hard- 
ness 


S 2 s s s s s 


ilii 


3SSSS« i 




Q 
55 


"^fii 


M o vo un .i) Ov c- 


^ 




> 

< 


ii 


S ^ ^ ;^ ^ ^ "^ 




stand- 
ard 
Brinell 
hard- 


5; 00 CO TO " TO S 




SB'S" 


S o\ S o ^ O^ o 


o 


o 

S 
55 


Sfll 


■^1- r^ -H o m -K vo 


en as 00 d c! en o 


< 


a" 






stand- 
ard 

Brinell 
hard- 
ness 


2 ? s s s s g 


.ilii 


3 s 5 E s s E 




n 
55 


m 


,r * .n CM -. 00 ~ 


+ - + 


< 


is 


_. o m -^ n 




stand- 
ard 

Brinell 
hard- 
ness 


sssssss 




TO O O O O TO a> 


< 


'^fii 


rt O 0\ rg oi O m 


< 


So 


'* O -* .-. Tf O <M 




KSSSSKS 


.i.sii 


SSS3SS3 





O-HOOOOOOOUICT. 


i/i 


+ 


> 

< 


-, -H " „ qi 


sSsSEssS 




Rsgggsaffi 


»o-.^o,^r^m 


- 


'»E3C:2"SS- 


< 


.-. ro fQ 12 ^ f3 " 


-.O-OCTifOOvf^i-. 




voiot^SinlrtSvo 


voi/iyacotno'coo 


o 


oo-.-..a>'MorM<M 


< 


+ 


??jSS5?°m5 


fM'Oro.*-..MiOr^ 


4.7 
3.7 

U.8 
10.5 
17,6 

3.7 
13.0 

2.2 


< 


■^m<j^<x>mt^'^<-3 






SSaSESHRS 




o. 


+ ~ '^ 


< 


+ "°"""''^' 




1 „,^,.HC^^.M^-0 


S??oo^^r2SSS 





tawdon 1 Hardness and Grain Size of Steels 

hmctw-Gil I 



58 I 




eoo 



900 



V,.A specimen ^vas heated six hours at the temperature itMRat.-,! an.l le.l in tire a,r 




F:o 19 -■■S,a,uU„r' Bn.ull l.a„l,u.^ of tl.e five steels after grain grc 
Hach.pecin,er,.asheateas.hnursattheter,.peratt,.einr.icateda„dcoo,edinthe.u™ace 



582 Scientific Papers of the Bureau of Standards \Voi. 16 

The purpose of determining the Brinell hardness by the two 
methods previously described was to show, if possible, the hard- 
ness of the grains as individuals or in small aggregates as com- 
pared Avith the average hardness of the material when measiu-ed 
in the usual manner. The results obtained by Portevin " on 
coarsely grained cast metals and alloys indicates that the hard- 
ness of individual crystals is not a constant value even for the 
same specimen, but it depends upon the relation of the surface 
bearing the Brinell impression to the internal orientation of the 
crystal. For convenience in reference tlie results of the determi- 
nations of hardness by the two methods are summarized in Table 6. 
With but very few exceptions (8 per cent of the total number of 
determinations), the micro-Brinell hardness is somewhat higher 
than that obtained in the usual manner. This is in agreement 
with the results obtained by Goodale and Banks '- upon cartridge 
brass, although the results here obtained by the two methods are 
in much closer agreement than those referred to. The fact tliat 
the hardness as obtained by the use of the small testing set is 
somevv'hat higher than that determined by the usual type of appa- 
ratus upon the same material is not necessarily to be interpreted 
as being due to a characteristic behavior of single grains (or small 
aggregates of grains) as compared with the combined effect of a 
large number of grains. It is, at least in part, to be attributed 
to a difference in behavior of the two types of apparatus, charac- 
teristic of the two methods. The average difference between the 
"micro-" and the "standard" Brinell hardness number is some- 
v.'hat greater for the steels of higher carbon content than for those 
low in carbon, particularly in the air-cooled specimens. It is 
impossil^le, however, to trace any clear and definite relationship 
between the differences existing between the two sets of hardness 
numbers and the grain size of the corresponding materials. 

In Fig. 20 are given micrographs to illustrate the fact that no 
appreciable and systematic difference in hardness measvued by 
the micro-Brinell testing apparatus as related to the number and 
size of grains covered l)y the impression could be detected. Im- 
pressions lying entirely Avithin the limits of a single grain were 
found to be essentially of the same size as those taken on the same 
specimen, but overlapping several crystals. The fact that the 
pearlite grain is not uniform throughout but consists of clearlv 

'■ A. Portevin, Hardness Tests on Individual Crystals, Rev. de Met., 12. p. 94; 1915. Also, The me- 
cbanical Anisotropy of Coarse-Grained Metals and Alloys and the Brinell Test. C. R., 160, p. 344; 1915. 
'■' Proc. A. S. T. M.. 1», Pt. II, p. ts7; 1919. 






Hanhicss and Grain Size of Steels 



583 



defined divisions which may vary very considerably among them- 
selves in size and in orientation is probably largely responsiljle for 
this. The micrographs of V\g. 20 also illustrate well the fact that 




Fig. 20. --Relation of the imf'icssion of ilic ball in the miao-Biiiull kH tu Ihc gunn cf 
the mclal of steel C, heated six hours at JII2° C and air-eookd; X/S {reduced from X '"" ) 

(a) — At iunction of four grains; (ft) — At junction of two grains; (c) — Within one grain, near boundary; 
(tl) — Entirely -vvilhin one grain. All impressions indicate tire same hardness number. Etched with 2 per 
cent alcoholic nitric acid solution 

the pearlite grains, particularly \vhen large, must each be con- 
sidered as an aggregate ratlier than as a single unit in their 
properties. 

The influence of the very pronounced increase in grain size 
which occurs in the steel when in the gamma condition upon the 



584 Sciciiiific Papers of the Bureau of Standards [Voi. t6 

hardness is indicated in all of the diagrams of Figs. 16 to 19, in- 
clusive. A sharp drop in the hardness curve corresponds to this 
rapid increase of grain size. The cun>'es also illustrate other 
features of interest concerning the hardness. The five steels 
naturally divide themselves into two classes. The two of the 
lower carbon content (0.07 and 0.19 per cent) are quite similar 
in their behavior; when in the annealed state (that is, cooled in 
the furnace) the tendency is for the hardness to remain rather 
uniform, aside from the drop which occurs as a result of the in- 
crease in grain size. The effect of cooling the material more 
rapidly — that is, in air — is to accentuate this drop and to raise 
the hardness generally throughout. It is of particular interest 
that the maximum temperature at which the specimen is heated, 
provided this does not exceed that at which the pronounced in- 
crease in grain size occurs, is quite negligible as to the degree of 
hardness produced upon air cooling. The three steels of higher 
carbon content (0.46, 0.70, and 1.12 per cent) form a group with 
some striking characteristics in common. The general tendency 
is for all of these materials to increase in hardness upon anneal- 
mg; cooling in air accentuates this tendency to a very marked 
degree. The drop in hardness corresponding to the pronounced 
increase in grain size which occurs is noticeable in all the speci- 
mens; in spite of this drop, however, the general tendency for an 
increase in hardness with increasing temperati.u"e of annealing is 
still marked. The maximum temperature to which the speci- 
men is heated before cooling either in the funiace or in the air 
is a factor of importance in determining the hardness. This is 
due, without doubt, to the larger percentage of carbon in the 
specimens comprising this group as compared with those of the 
former one. The rate at which the steel is cooled after the long 
period of heating is a factor of prime importance in determining 
its hardness. Figs. 12 and 13 show the structural condition of 
the hardening constituent (pearlite or sorbite) due to the rate at 
which the material is cooled. Although the specimens which 
were cooled in the furnace have been given the same designation — 
that is, "furnace cooled " — it is probable that the rate of cooling 
was not always the same, but varied somewhat according to the 
maximum temperature to which the furnace was heated. It 
may be inferred from the micrographs (Figs. 9 and 10) that a 
slight decarburization occurred in specimen E at the high tem- 



^imiZcu] Hardness ami Grain Size of Steels 585 

peratures, in spite of the precautions taken. Such a sHght de- 
carburization, however, would only decrease the slope of the 
cur\-e slightly and not affect the general results obtained. 

The increase in the size of the aggregates of ferrite in the speci- 
mens heated at the higher temperatures appears to have but 
little effect upon the micro-Brinell hardness determinations. A 
slightly greater tendency toward scattering at the higher temper- 
atures may be observed in the plotted data of the curves, but 
this is all. 

The pronoimced tendency shown by all the steels toward an 
increase in hardness at the highest temperature used as com- 
pared with the same steel heated at the next lower temperature 
is very striking iKigs 16 to 19). Each specimen shows a marked 
rise in hardness immediately following the pronounced drop, 
previously attributed to the increased grain size. The material, 
although "overheated," has not been "burnt," so the increase 
of hardness can not be attributed to this cause. It is, however, 
approaching a condition analogous to that resulting from casting, 
and evidently this pronounced tendency for an increase in hard- 
ness is due to some condition similar to that within a casting, 
which does not reveal itself in the strticture alone. 

While it is realized that the factors discussed do not account 
entirely for the phenomena observed as regards Brinell hardness 
of the materials studied, still it is evident that the conclusion is 
warranted that grain size is a factor of minor importance. The 
rate of cooling, together with the accompanying structural and 
other changes due to the transformations which occur within the 
metal, are of far greater import. There is no simple relation be- 
tvv-een grain size and Brinell hardness, as in the metals and alloys 
of simpler structure. 

VI. GRAIN GROWTH UPON ANNEALING AFTER COLD 
WORKING '^ 

The second method fur producing coarsely grained material, 
previously referred to on page 559, consisted in heating specimens 
which had been deformed by cold v/orking. The results obtained 
are included here for their suggestiveness rather than as a com- 
plete study of the subject of grain growth after strain. The 
rapid recrystallization of iron and mild steel, by which extremely 
coarse crystals can be produced upon annealing after permanent 

" The authors are indebted to R. W. Woodward for aid in making the mechanical tests discussed in 
this section. 



586 Scientific Papers of the Bureau of Standards [Voi.16 

deformation of the structure, is well know and has formed the 
subject of several extensive researches. Among these may be 
mentioned that of Sauveur," who, following the suggestions of 
Charjjy'^ and Le Chatelier,'" has shown that rapid recrystalliza- 
tion occurs in iron only when the deformation has been carried to 
a certain "critical" degree. The most extensive study of the 
subject has been made by Chappell,'' who differs in his conclu- 
sions from Sauveur. 

The specimens used for the production of the coarsely granular 
areas were somewhat similar to those of Chappell, who employed 
Fremont's device for obtaining differential stress within the same 
bar.'* These were tapered tension specimens of the size and form 
shown in Fig. 21. As materials, two bars of each of the composi- 
tions A, C, and D (Table i) were used; these were annealed for 
two hours at 650° C and cooled in the furnace. This was for the 
purpose of removing the effect of the cold working which the sur- 
face of the specimen received during the necessary machine work 
and which might affect the sub.sequent recrystallization. This 



U M U I.' I- I' 



Fig. 2i. — J'tUiioii ipvuiiwii kmJ J'oi dijhiciilially ilniiiiiiiii the ittrl before ainicaling 

effect may be of consideraVjle magnitude, as has been previously 
shown. '^ The specimens, after polishing the sloping surface 
suitably for microscopic examination, were stressed in tension 
until rupture occurred; one observer closely watched the pol- 
ished specimen and the stress was recorded as soon as a roughening 
of the sm^ace appeared at each of the marked points (Fig. 21). 
In this way the necessary stress required to slightly deform the 
specimen and give rise to the diagonal flow marks, or " Luder's 
lines," could be measured rather accurately. The maximum 
stress carried by the bar at each of the marked subdivisions was 
also calculated from the measured maximum stress borne by the 
specimen. After microscopic examination ot the surface of the 
strained bars for the occurrence and distribution of slip bands as 

" Albert Sauveur. Crystalline Growth of Ferrite Below Its Thermal Critical Range. Proc. Int. Asso. 
Test. Mat.. 6th Cong.. 2; igu. 

'■■ Georges Charpy. Sur la Maladie de rficrouissage, Rev. de Met.. Meniuirs, i . p. 6^5; 1910. 

'^ Henri Le Chateher, Notes de Metallographie. Rev. deMet.,8. p. 367; 1911. 

" C. Chappell. RecrystaUization of Deformed Iron, J. Iron and Steel Inst., 89, No. i. p. 460; 1914. 

IB Frdmont, Mesuredela Limite filastique desM^taux, Bull. Soc. Encouragement Ind. Nat,. «5. Pt. 11, 
p. 363: 1903. 

''' H. S. Rawdon, B. S. Tech. Papers, No. 60. 



j^^n^ci] Hardness and Grain Size of Steels 587 

distinct from the Luder's lines, the specimens were annealed for a 
period oi six hours at 686° C (680-692) ; that is, slightly below the 
critical range. One bar, A-i , C-i , and D-i , ot each set was cooled 
in the air, the others, A-i i , C-i i , and D-i i , were allowed to cool 
in the furnace. It has recently been shown by Hanson '" that the 
time recjuired for the recrystallization of strained metals to occur, 
at least for the soft metals, aluminum, zinc, etc., is very short. A 
period of a few minutes at the chosen temperature is sufficient. 
The object of the long ])eriod of annealing was to remove the hard- 
ness due to the cold work, as well as to permit the maximum grain 
growth to occur. It was hoped that in this way specimens which 
varied only in grain size along the length of the bar might be 
obtained. 

Tlie rcsidts of the hardness measurements of the strained-and- 
annealed specimens, together with the data of the tensional 
stressing, are summarized in Table 7. The appearance of some 
of the bars after recrystallization is shown in Fig. 23. 

In the case of the low -carbon steel f)nly (A-i and A-ii, 0.07 
per cent carbon) was the grain found to increase in size during the 
treatment given. In the specimens of higher carbon content 
(0.40 and 0.70 per cent) no appreciable change could be detected. 
This confirms Chappell's observations ^' in this respect. The 
hardness measurements of specimens A-i and A-ii are in general 
conformity with those of the bars of which the grain was coarsened 
by heat alone (Tables 4 and 5) ; that is, a pronoimced increase in 
grain size is accompanied by a lowering of the Brinell hardness. 
In the other specimens (C and D) the hardening effect of the 
straining was removed by the prolonged heating, and the speci- 
men showed no pronounced differences in hardness along its 
length. In some cases the end of the bar which was most severely 
strained was found to be somewhat softer than the other, although 
no perceptible increase has occurred in the size of the grain in such 
specimens. Although the steels were heated at a temperature 
considerably below that of the Ai transformation, all the speci- 
mens cooled in air were noticeably harder than those allowed to 
cool in the furnace. 

The results of the tension test tlirow some additional light upon 
the conditions necessary for the rapid recrv'stallization of soft steel 
upon annealing after strain. 

-" D. Hanson. Rapid Recrystallization in Deformed Nonlerrous Metals, J. Inst. Metals. 20. No. .', p. 141; 
191S. 
"J. Iron and Steel Inst., 89. No. i, p. 460; 1914, 











>* * 










3' 



'■*/ <i^/ 



K^^' 

*-?*-^- 



5S8 



1 IG 2 J — I oiitUtion of the surface of tapered tension 
sptiimcn if o.oy per cent carbon steel A after 
straming; )<,JCio 

The figures indicate the position of the micrograph on 
the bar {Fig. 21). Xo slip hands were found beyond marlc 
No. ^. N'ot etched 






Hanhicss a)id Grain Size of Steels 



589 




Fig. 23.— Gni/>/. yuni'th pnxluccd by the low-lew paalitre anncalinq of sUrl A aflci 
strain; y,2 

\ in tension till rupture occurrcil. 



The specLmeus were annealed six hours at 6Si6*C after bein 
The numbers correspond to the subdivisiims (Fie. 21). M 
applied stress which was ;6.8 per cent greater than the elasti 
with 2 per cent alcoholic nitric acid 



■limii 



1 A-i corresponds in 
2; pi.T cent, Ktched 



590 



Scientific Papers of the Bureau of Standards [Voi.i6 

TABLE 7. — Brinell Hardness of Steel, Annealed after Straining 





Stress 






1 






Stress 












to per- 




Micro- 
Brinell 
hard- 


Stand- 






to per- 






Stand- 


steel 1 


Divi- 


nently 


Maxi- 
mum 


ard 
Brinell 


Steel » 


Divi- 
sion (< 


ma- 
nently 


Maxi- 
mum 


Brinell 
hard- 


ard 
Brinell 




sion 




stress 


hard- 




deform 


stress 


hard- 






mate- 
rial 






nessd 






mate- 
rial 






nessd 




1 
Lbs/in = 


Lbs/in = 










Lbs/in •- Lbs/in = 








1 
1.5 


27 540 


49 280 
43 430 


79 
84 


75 




1 
1.5 


46 385 
45 530 


97 370 
89 045 


148 
162 








163 




' 2 


26 350 


40 325 


112 


77 




2 


47 800 


82 730 


158 


150 






2,5 


27 180 


35 640 


117 


81 




2.5 


50 600 


77 920 


156 


156 






3 


27 800 


35 670 


121 


89 




3 


47 870 


74 200 


154 


150 






'3.5 


27 180 


33 985 


119 


94 




3.5 


51 960 


70 480 


153 


147 




4 


27 150 


32 530 


127 


95 




4 


51 140 


67 525 


148 


146 




1.5 




31 255 


126 


94.5 




4.5 


49 550 


64 450 


151 


147 




5 


27 100 


30 010 


127 


94 




5 


46 020 


61 850 


154 


144 




5,5 




28 930 


126 


97 




5.5 


47 070 


58 340 


153 


140 




6 


25 950 


27 950 


124 


96 




6 


44 750 


57 080 


155 


140 




6.5 


26 430 


27 030 


127 


95 




6.5 


45 800 


55 020 


161 


140 




7 




26 205 


127 


94 




7 


46 320 


53 290 


162 


144 




7.5 


25 200 


25 450 


126 






7.5 


45 150 


51 390 


169 


142 


. 




26 790 






Averate 


47 560 








Average 




.. . . . 














1 
1. 5 




48 800 
45 020 


74 
31 


74 


D 1 


1 
1.5 


56 940 jU4 260 
108 660 


154 
162 


166 






162 




12 


26 650 


42 075 


86 


76 




2 


56 320 


95 920 


167 


163 




1 


2.5 


26 480 


41 140 


83 


74 




2.5 


51 165 


91 330 


164 


161 






3 


28 390 


37 790 


107 


76 




3 


51 160 


86 010 


165 


160 






3.5 


27 700 


36 120 


112 


79 




3.5 


47 540 


81 670 


165 


161 




/4 


27 010 


34 240 


111 


81 




4 


51 890 


77 655 


167 


159 




4.5 


27 720 


32 815 


110 


89 




4.5 


51 490 


74 500 


170 


159 




5 




31 340 


109 


88 




5 


51 800 


72 000 


173 


159 




5.5 


27 280 


30 040 


108 


87 




5.5 


52 240 


68 380 


182 


159 




6 


26 500 


28 860 


111 


87 




6 


52 240 


65 875 


181 


159 




6.5 


26 820 


27 780 


109 


86 




6.5 


52 420 


63 490 


184 


160 




7 


25 960 


26 730 


114 


86 




7 


52 360 


61 250 


184 


160 




7.5 


25 650 


25 740 


116 


88 




7.5 




59 120 


183 


165 


A 




26 930 




1 




52 290 




























1 
1,5 


51 330 
48 510 


99 290 
84 140 


156 


147 
156 


1 
1.5 


56 735 


115 450 
106 540 


166 


165 






155 




2 


46 570 


82 410 


157 


155 




2 


52 030 


98 835 


164 


160 




2.5 


47 000 


77 440 


158 


150 




2.5 


51 200 


93 710 


164 


159.5 




3 


47 160 


73 640 


166 


153 




3 


51 860 


88 060 


167 


157.5 




3.5 


47 060 


70 260 


166 


147 




3.5 


52 120 


84 960 


165 


158 




4 


47 530 


67 390 


154 


147 




4 


52 090 


80 700 


165 


153 




4.5 


47 310 


64 585 


153 


152 




4.5 


52 610 


77 270 


168 


157.5 




5 


46 620 


61 875 


152 


149 




5 


52 000 


74 300 


168 


161 




5.5 


47 750 


59 725 


160 


147 




5.5 


53 870 


73 270 


176 


161 




6 


47 270 


57 455 


165 


152 




6 


51 920 


68 490 


172 


161 




6.5 


47 180 


55 530 


168 


149 




6.5 


51 930 


65 870 


178 


160 




7 


47 095 


53 574 


165 


150 




7 


51 760 


63 650 


176 


161 




7,5 


46 880 


51 730 


168 


147 


^ 


7,5 




60 890 


181 


163 


Averaee 




47 520 








52 510 








1 



















" Specimens 
Specimens A-i 
^See Fig. 21 
*■ IS kgload. 
<* 500 kg load 



I, C-i, D-i (composition, Table i) were cooled 
>ii, D-ii (composition. Table 1 ) were cooled in tl 



- after the final anncalinj 
after the final annealing. 



/. 



ich ball, pressure applied for ^o seconds. 
g lutt-^i. KJ inm ball, pressure applied for 30 seconds. 
<' Zone of pronounced cristal growth. Fig. 23. 
,/' Zone of pronounced crystal growth, Fig. 23. 



j!mf,','"G,i\ Hard>tcss and Grain Size of Steels 591 

The stress necessary for the apjjearance of the "flow" or 
"stress" lines at different positions along the tapered specimen 
gives a rather accurate measurement of the "elastic limit," as 
defined in this way. The microscopic examinatioii of the surface 
of the specimen after straining shows that the metal nnist be 
stressed considerably more than this value in order to produce 
the distortion within the crystal familiarly known as "slip l)ands." 
Only in those portions of the bar in which slip bands were found 
was there an increase of crystal size on annealing. The converse, 
however, was found by observation not to be true. A com- 
parison of the maximum stress borne by the bar at different points 
along its length with the "elastic limit" as defined by the test 
demonstrates that, for pronounced coarsening of the grain at the 
temperature used, the "elastic limit" must be exceeded by 
amounts varying from about 25 to 55 per cent. In A-i the region 
of pronounced growth corresponds to a stress exceeding the 
"elastic limit" by 26.8 to 52 per cent; in A-i i , 27 to 56 per cent. 
In both cases, the zone of crv'stals of maximiun size corresponds 
closely to the lower percentage given aljove. It is evident that 
when the first roughening of the surface appears — that is, when 
the Luder's lines (45° flow lines) are formed — the individual 
crystals of the material have not yet been permanently deformed 
to any appreciable extent, as no slip bands were found above 
division 5 for the low-carbon steel (Fig. 22), or beyond division 3 
for each of the other steels (Fig. 24) . The material was perma- 
nently strained, however, along the tapered length sufficiently to 
])roduce the characteristic appearance due to the Luder's lines 
(P'ig. 25) and to permit a determination of the stress at the " elastic 
limit" being made (Table 7). 

When the metal is stressed considerably more than the limits 
given above, so that the crystals are very badly distorted, as 
shown by the pronounced roughening of the surface by the slip 
bands (Fig. 22), the grain size after annealing, although consider- 
ably larger than originally, is still far below the maximum attained 
in other parts of the specimen. 

VII. SUMMARY 

I . The Brinell hardness was determined for five steels varying 
in carbon content from a very low value to somewhat above 1 
per cent. Each of the steels was treated so as to produce wide 
variations in grain size, and the hardness was determined in each 
condition. 



592 



Scientific Papers of the Bureau of Standards 



lV,il.l6 



2. Upon heating for six-hour periods no very appreciable in- 
crease in the grain size occurs until the Acj transformation in the 
steel has occurred. The change in grain size often appears to l)e 
a ver\' abrupt one ; that is, it takes place within a rather narrow 
range of temperatiu"e. 




Fig. 24. — Condition of tapered tension spccinwn C after ilraiiii 
(Table y); X7 5 {reduced from y.ioo) 

The figures refer to the position on the bar (Fig. 21). Slip bands were found 
the surface as far as position 3. No grain growth upon annealing occurred, h 
as a result of this deformation. Etched with 2 per cent alcoholic nitric acid (for the 
micrographs at the right, olliers inictched) 

3. Two methods were used for obtaining the Brinell hardness, 
one of which was intended to give the hardness of individual 
crystals or small aggregates as distinct from the average hardness 
of the material. 

4. The results of the two methods show no appreciable or con- 
sistent difference between the hardness of small groups of crystals 
and the average hardness for the steels investigated. 



J„ne,HUi,li 



Hanl)uss and Grain Size of Steels 



593 



5. Although it ^vas iinpossiljle to olitain an accurate uumerical 
grain-size determination for many of the specimens, tlie micro- 
graphic examination indicates that there is no simple and direct 
relation between grain size and Brinell hardness number for car- 
bon steels. A very pronounced increase in grain size is usually 
accompanied by a decrease in hardness. 
On the whole, however, grain size ap- 
pears to be a factor of minor impor- 
tance in determining the Brinell hardness 
of carbon steels of the types investigated. 

6. The general effect of heating the 
steel — that is, ujion the pr()])erties of 
the metal after cooling — is to harden it 
appreciably. This increase is noticealjie 
in spite of a pronounced drop in hard- 
ness which accompanies an abrupt in- 
crease in grain size. This tendency 
toward hardening upon heating is ni)t 
shown by low carbon steels to au} 
extent, thus suggesting that this change 
in hardness is not a function ot the 
grain size. 

7. The rate at which steels are cooled, 
and conse(|uenth' the structural condi- 
tion of the hardening constituent, affects 
the hardness much more than any other 
factor. 

S. The hardness measurements upon 
materials in ^vhich a pronounced differ- 
ential grain groAvth has been produced b}- 
lo\\ -temperatvire annealing after strain- 
ing the metal are in general accord with 
the results obtained upon the same steels in Avhich the grain was 
coarsened by heat alone. 

9. Incidental to the study of the hardness of steels coarsened 
by annealing after permanent strain, some data were obtained 
relative to the magnitude of the necessary stress required to cause 
pronounced grain growth upon aianealing such strained metal 
below the Aci transformation tempera tm-e. 

Washington, April 2S, 1920. 



f '•■'■: v^lj^H 


7.5- 



Fig. :^ —Porllon of the tattered 
tension st>cci>mii.sla-l A , from 
Div. _- /<) ;.5 (/•'/,7. 21) after 
straining 

The 45° How lines (Ludcr's liius) are 
very prominent ; some rouyhcnmg 
due to slip bands may also be seen. 
Specimen is unetched and is slightly 
larger than natural size 






003 



129 8671 



